Conventionally, when a free curved surface is machined, after tool path data represented by approximating, with minute line segments, from straight lines, arcs, and curves, a path formed by imaginarily moving a tool in contact with a free curved surface of a machining target is created using computer aided design (CAD) and computer aided manufacturing (CAM) apparatuses, this tool path data is given to an NC machine tool to perform cutting.
The tool path data output from the CAD and CAM apparatuses is mainly described as an NC program. The NC program is input to an NC apparatus included in the NC machine tool.
The NC apparatus reads and interprets the NC program to thereby generate a tool path, which is a path on which a tool moves, and creates interpolation data obtained by interpolating the tool path for each of interpolation periods.
The NC apparatus controls tool axes of tools provided in the NC machine tool with the created interpolation data, moves the tools to desired positions, and performs machining.
A procedure for generally generating tool path data for machining a free curved surface is explained. In the following explanation, the free curved surface is referred to as “machining curved surface”.
First, from a shape of a machining curved surface that should be machined in a machining target, an ideal path requested when a tool is moved in contact with the machining curved surface is calculated.
Subsequently, an allowable error is given. Tool path data is generated by approximating a tool path to a minute line segment such that a maximum error between the tool path and the calculated ideal path is equal to or smaller than the given allowable error.
The tool path data is generated on the basis of the procedure explained above.
According to the procedure explained above, the generated tool path data has a maximum error equivalent to the allowable error with respect to the ideal path. Calculation errors of the CAD and CAM apparatuses are present in the error. Therefore, tool path data of a minute line segment unrelated to the direction of the ideal path is sometimes included in the generated tool path data.
As a result, when machining is performed using the tool path data, in some case, movement of the tool unintended in the ideal path occurs and scratches on a machining surface occur.
To accurately move the tool with respect to the machining curved surface, it is necessary to reduce the allowable error given during the generation of the tool path data to be extremely small. However, in that case, problems described below occur.
(1) A time for creating and outputting the tool path data with the CAD and CAM apparatuses increases.
(2) A data amount of the tool path data input to the NC apparatus is enormous.
(3) Machining speed cannot be increased because the data amount of the tool path data is enormous.
(4) Work efficiency decreases because of these problems.
Therefore, it is undesirable to reduce the allowable error given during the tool path data generation to accurately move the tool with respect to the machining curved surface
As a solving method in such a case, in Patent Literature 1, tool path data is corrected using the shape of a machining target object. In the following explanation, the shape of the machining target object is simply referred to as “machining shape”.
Specifically, Patent Literature 1 discloses a method of dividing the tool path data into a plurality of divided tracks, calculating, as axis control data, any tool positions on the divided tracks and temporal changes of tool moving speed in axial directions on the divided tracks, comparing a tool position in machining work calculated on the basis of the axis control data and a machining shape of the work, determining whether a tool in an intruding state into the machining shape, and, when it is determined that the tool is in the intruding state, correcting the position of the tool in the intruding state using continuous preceding and following tool positions such that the intruding state is avoided.